problem is, and why some will tell you not to grease, is because if you go overboard, you're gonna sling grease all over your clutch disc, ruining it.
throwout bearings come pregreased with a white lithium type grease and really shouldn't require anything additional besides what's already on it.
I've always installed everything bone dry, and as is, however, on my most recent project, I put a very, very slight additional coating on the inner part of the throwout bearing.